Tree-ring stable isotopes are insightful proxies providing information on pre-instrumental climate fluctuations, yet the variability of these data within a tree trunk has not been fully explored. Here, we analyze longitudinal and circumferential changes in tree-ring delta C-13 values from 1991-2010, considering seven height levels from 1 to 13 m above ground and six sampling directions (radii) separated by 60 degrees around the stem. The disk samples were taken from a 360-year old European larch (Larix decidua Mill.) that grew at 1675 m above sea level in the Simplon Valley, Switzerland. Results show that the circumferential delta C-13 variability, defined as the difference between the minimum and maximum isotope values within a single ring at a certain height, ranges from 0.5 to 2.8 parts per thousand. These differences appear substantial as they match the range of year-to-year variations retained in long tree-ring delta C-13 time series used for climate reconstruction. The assessment of longitudinal variability demonstrated a systematic change of similar to 0.1 parts per thousand m(-1) towards isotopically heavier (less negative) delta C-13 values with increasing tree height, likely reflecting a vertical gradient towards isotopically heavier needle tissue due to changing microclimatic conditions and CO2 stratification within the canopy. Calibration against regional climate data indicates no substantial signal changes in delta C-13 values within the trunk. We conclude that the longitudinal isotope gradient adds uncertainty to long delta C-13 chronologies derived from subfossil material of unknown (and changing) sampling heights. The large circumferential variability recorded in the sub-alpine larch suggests that more than two cores are needed to analyze absolute delta C-13 values representative for each tree.